A wave-exposed sea stack and a protected boulder beach at
Cayucos Point, California, were compared in terms of their intertidal biota on
17-18 February 1973. The major differences between the two sites appear to be
due largely to differences in the shearing forces of waves and habitat structure.
The mosaic of crevices, rivulets, and angled substrates in conjunction with a
broad gradual slope and reduced wave action at the boulder beach habitat
resulted in a predominance of macrophytes and a zonational pattern related to
both horizontal location on the shore and vertical tidal level, while sessile
macroinvertebrates with zonal patterns closely correlated to tidal height
dominated the sea stack. Upward shifts in comparable vertical zones at the sea
stack were clearly correlated with increased wetting higher on the shore due to
waves and splash, in agreement with similar findings by other workers. The
most abundant macrophytes at both sites were blue-green algae and Endocladia
muricata,-althoughtheotherabundant.speciesweredifferentat-each site. Eive
sessile macroinvertebrates (Mytilus californianus, Chthamalus fissus, C. dalli,
Balanus (Balanus) glandula, and Pollicipes polymerus) dominated the sea stack,
while only three sessile speci((s (Anthopleura elegantissima, C. fissus, and C.
dalll) were prevalent on the boulder beach. Of the mobile macroinvertebrates,
Tegulafunebralis was the most numerous species at the boulder beach whereas
the limpets Acmaea (Collisella) scabra and A. (Collisella) digitalis occurred
most abundantly on the sea stack. Although a greater number of taxa and
higher species richness values were recorded at the boulder beach, the evenness
index and Shannon's index indicated a higher diversity on the sea stack. At the
boulder beach, 12 species assemblages were defined by cluster analysis, while
only 6 such groups were identified on the sea stack. The boulder beach macrophytes
contributed approximately one-third more to total community primary
production than did those of the sea stack (169.7 versus 116.5 net mg C m-2 h-1),
due mainly to the greater cover and concomitant production by Cyanophyta
and fucalean Phaeophyta.